6Renewable Energy

This chapter reviews the status of renewable resources as a source of usable energy. It describes the resource base, current renewables technologies, the prospects for technological advances, and related economic, environmental, and deployment issues. While the chapter’s focus is on renewables for the generation of electricity, it also includes short discussions of nonelectrical applications. The use of biomass to produce alternative liquid transportation fuels is not covered in this chapter but rather in Chapter 5.

CURRENT STATUS OF RENEWABLE ELECTRICITY

Generation of Renewable Electricity in the United States

Renewables currently account for a small fraction of total electricity generation. According to the U.S. Energy Information Agency (EIA, 2007), conventional hydropower is the largest source of renewable electricity in the United States. Representing about 71 percent of the electric power derived from renewable sources, hydropower generated 6 percent of the electricity—almost 250,000 GWh out of a total of 4.2 million GWh—produced by the electric power sector in 2007.1

The nonhydropower sources of renewable electricity together contributed 2.5 percent of the 2007 total. Within this group, biomass electricity generation (called

1

The electric power sector includes electricity utilities, independent power producers, and large commercial and industrial generators of electricity.

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6 Renewable Energy
T
his chapter reviews the status of renewable resources as a source of usable
energy. It describes the resource base, current renewables technologies,
the prospects for technological advances, and related economic, environ-
mental, and deployment issues. While the chapter’s focus is on renewables for the
generation of electricity, it also includes short discussions of nonelectrical applica-
tions. The use of biomass to produce alternative liquid transportation fuels is not
covered in this chapter but rather in Chapter 5.
CURRENT STATUS OF RENEWABLE ELECTRICITY
Generation of Renewable Electricity in the United States
Renewables currently account for a small fraction of total electricity generation.
According to the U.S. Energy Information Agency (EIA, 2007), conventional
hydropower is the largest source of renewable electricity in the United States. Rep-
resenting about 71 percent of the electric power derived from renewable sources,
hydropower generated 6 percent of the electricity—almost 250,000 GWh out of a
total of 4.2 million GWh—produced by the electric power sector in 2007.1
The nonhydropower sources of renewable electricity together contributed 2.5
percent of the 2007 total. Within this group, biomass electricity generation (called
1The electric power sector includes electricity utilities, independent power producers, and
large commercial and industrial generators of electricity.
271

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272 America’s Energy Future
“biopower”)2 is the largest source, having produced 55,000 GWh in 2007. Wind
power and geothermal supplied 32,000 GWh and 14,800 GWh, respectively, dur-
ing that year. Except for wind power, none of these sources has grown much since
1990 in terms of either total electric power production or generation capacity.
The largest growth in the use of renewable resources for electricity genera-
tion is currently in wind power and, to a lesser extent, in solar power. Wind
power technology, having matured over the last two decades, now accounts for
an increasing fraction of total electricity generation in the United States. Though
wind power in 2007 represented less than 1 percent, it grew at a 15.5 percent
compounded annual rate over the 1990–2007 period and at a 25.6 percent com-
pounded annual growth rate between 1997 and 2007. Wind power supplied
almost 6,000 GWh more in 2007 than it had the year before. According to the
American Wind Energy Association, an additional 8,300 MW of capacity was
added in 2008 (AWEA, 2009a), representing an additional yearly generation of
25,000 GWh assuming a 35 percent capacity factor.3 By the end of 2008, the
overall economic downturn had caused financing for new wind power projects
and orders for turbine components to slow, and layoffs began in the wind turbine
manufacturing industry (AWEA, 2009a). Thus new capacity in 2009 recently
looked to be considerably smaller than in 2008. However, AWEA (2009b)
recently reported that 2.8 GW of new wind power generation capacity was
installed in the first quarter of 2009. Further, analysis of the American Recovery
and Reinvestment Act (ARRA) of 2009 shows that by 2012 wind power genera-
tion will more than double what it would have been without the ARRA (Chu,
2009).
Central-utility electricity generation from concentrating solar power (CSP)
and photovoltaics (PV) combined was 600 GWh in 2007, just 0.01 percent of the
U.S. total—a fraction that has been approximately constant since 1990. However,
this estimate does not include contributions from residential and other small PV
installations, which now account for the strongest growth in solar-derived electric-
ity. Installations of solar PV in the United States have grown at a compounded
annual growth rate of more than 40 percent from 2000 to 2005, with a genera-
2Biopower includes electricity generated from wood and wood wastes, municipal solid wastes,
landﬁll gases, sludge wastes, and other biomass solids, liquids, and gases.
3The capacity factor is deﬁned as the ratio (expressed as a percent) of the energy output of a
plant to the energy that could be produced if the plant operated at its nameplate capacity.

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Renewable Energy
tion capacity of almost 480 MW that, assuming a 15 percent capacity factor, pro-
duces approximately 630 GWh.
Current Policy Setting
At present, electricity generation from non-hydropower renewable sources is gen-
erally more expensive than from coal, natural gas, or nuclear power—the three
leading U.S. options. Thus policies at the state and federal levels have provided the
key incentives behind renewable sources’ recent penetration gains.
One such policy is the renewables portfolio standard (RPS), which typically
requires that a minimum percentage of the electricity produced or sold in a state
be derived from some collection of eligible renewable technologies. Given that
these RPSs have been developed at the state level, there are many different ver-
sions of them. The policies differ by the sources of renewables included (some
states specify conventional hydropower or biopower); by the form, timeline, and
stringency of the numerical goals; and by whether the goals include separate tar-
gets for particular renewable technologies. As of 2008, 27 states and the District
of Columba had RPSs and another 6 states had related voluntary programs. Wiser
and Barbose (2008) estimate that full compliance with these RPSs would result in
an additional 60 GW of new renewables capacity by 2025. Assuming a 35 percent
capacity factor, which means that the capacity produces electricity for approxi-
mately 3070 hours per year, an additional 180,000 GWh from renewable sources
would be generated. This is compared to the estimated total of 4.2 million GWh
generated in 2007.
Federal policies are also contributing to this era of strong growth in
renewable-energy development. The major incentive, particularly for wind power,
is the Federal Renewable Electricity Production Tax Credit (referred to simply
as the PTC), which provides a $19 tax credit (adjusted for inflation) for every
megawatt-hour (equivalent to 1.9¢/kWh) of electricity generated in the first 10
years of life of a private or investor-owned renewable electricity project brought
on line through the end of 2008.4 Congress most recently extended the PTC and
expanded incentives for 1 year in the Emergency Economic Stabilization Act of
2008 and the ARRA of 2009. These two bills together extend the PTC for wind
through 2012 and the PTC for municipal solid waste, qualified hydropower,
biomass, geothermal, and marine and hydrokinetic renewable-energy facilities
4After adjusting for inﬂation, the current PTC is 2.1¢/kWh.

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274 America’s Energy Future
160
140
Average Price of
Wind Power
without PTC
2005 Dollars per Megawatt-hour
120
100
Operating Cost of
Natural Gas Combustion Turbine
80
Average Price of
Wind Power with PTC
60
40
20 Operating Cost of Wholesale Price Range
Natural Gas Combined Cycle for Flat Block of Power
0
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
Year
FIGURE 6.1 Impacts of the PTC on the price of wind power compared to costs for
natural-gas-fired electricity.
Source: Wiser, 2008.
through 2013. Because of concerns that the current slowdown in business activ-
ity will reduce the capabilities of projects to raise investment capital, the ARRA
allows owners of nonsolar renewable-energy facilities to elect a 30 percent invest-
ment tax credit rather than the PTC. Figure 6.1 shows the impact of the PTC
on the price of wind power versus that of natural-gas-fired electricity, though it
should be noted that other current electricity sources, such as coal, hydropower,
and nuclear, have lower operating costs than do natural gas combined-cycle
plants.

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Renewable Energy
RESOURCE BASE
Size of Resource Base
The United States has significant renewable-energy resources. Indeed, taken col-
lectively they are much larger than current or projected total domestic energy
and electricity demands. But renewable resources are not evenly distributed spa-
tially and temporally, and they tend to be diffuse compared to fossil and nuclear
energy. Further, although the sheer size of the resource base is impressive, there are
many technological, economic, and deployment-related constraints on using these
sources on a large scale.
The United States has significant wind energy resources in particular;
Figure 6.2 shows their distribution across the country. The total estimated electri-
Wind Resource Wind Power Wind Speed Wind Speed
Power Potential Density at 50 m at 50 m at 50 m
Class W/m2 m/s mph
2 Marginal 200–300 5.6–6.4 12.5–14.3
3 Fair 300–400 6.4–7.0 14.3–15.7
4 Good 400–500 7.0–7.5 15.7–16.8
5 Excellent 500–600 7.5–8.0 16.8–17.9
6 Outstanding 600–800 8.0–8.8 17.9–19.7
7 Superb 800–1600 8.8–11.1 19.7–24.8
FIGURE 6.2 U.S wind resource map showing various wind power classes. Areas shown
in white have class 1 wind resources.
Source: NREL, 2007a.

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cal energy potential for the continental U.S. wind resource in class 3 and higher
wind-speed areas is 11 million GWh/yr (Elliott et al., 1991), greater than the 2007
electricity generation of about 4 million GWh. The 11 million GWh estimate was
obtained from point-source measurements of the wind speed at a height of 50
meters (m); the actual value could differ substantially (Elliott et al., 1986). On the
one hand, modern wind turbines can have hub heights of 80 m or greater, where
larger wind energy resources are likely available. On the other hand, computer-
model simulations of very-large-scale wind farm deployments have shown that an
agglomeration of point-source wind speed data over large areas can significantly
overestimate the actual wind energy resource (Baidya et al., 2004). Estimating the
upper-bound limit for extraction of the resource at 20–25 percent of the energy
in the wind field, and using the total domestic onshore wind electricity potential
value of 11 million GWh, an upper bound for the annual extractable wind electric
potential is perhaps 2–3 million GWh. This potential resource base is about half
of the current electrical power use in the United States, and significant offshore
wind energy resources also exist and increase the wind resource base considerably.
The solar energy resource also is very large indeed. Taking solar insolation to
be a representative midlatitude, day/night average value of 230 W/m2, in conjunc-
tion with the area of the continental United States of 8 × 1012 m2, yields a yearly
averaged and area averaged power-generation potential of 18.4 million GW. At
10 percent average conversion efficiency, this resource would therefore provide
1.6 billion GWh of electricity annually. For 10 percent conversion efficiency,
coverage of 0.25 percent of the land of the continental United States would be
required to generate the total 2007 domestic electrical generation value of 4 mil-
lion GWh. However, the solar resource is very diffuse and, as shown in Figure 6.3,
distributed unevenly across the country.
Additionally, the various technologies for tapping solar energy utilize differ-
ent aspects of sunlight. Because CSP, for example, can exploit only the focusable
direct-beam portion of sunlight, highly favored sites are located almost exclu-
sively in the Southwest. Further, because CSP can use only the direct-beam por-
tion of sunlight, energy input to the CSP plants falls to zero in the presence of
clouds. However, most designs today decouple energy collection from the power
cycle through the use of thermal storage, and thus the power output of the CSP
plant will not immediately fall to zero in the presence of clouds. A recent analy-
sis, which identified lands having high average insolation (>6.75 kW/m2 per
day) and excluded regions of such lands having a slope >1 percent or a small
(<10 km2) continuous area, estimated that CSP could deliver an average of

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kWh/m2/day
>9.0
8.5–9.0
8.0–8.5
7.5–8.0
7.0–7.5
6.5–7.0
6.0–6.5
5.5–6.0
5.0–5.5
4.5–5.0
4.0–4.5
3.5–4.0
3.0–3.5
2.5–3.0
2.0–2.5
<2.0
FIGURE 6.3 Solar energy resources in the United States.
Source: NREL, 2007b.
15–30 million GWh/yr of electrical energy, which is 4–7 times larger than the total
U.S. supply (ASES, 2007).
Flat-plate PV arrays can be distributed more widely than concentrated solar
power systems because flat-plate systems effectively utilize both the diffuse and
the direct-beam components of sunlight. Analyses of the total rooftop area that
would be suitable for installation of PV systems have produced resource estimates
ranging from 0.9–1.5 million GWh/yr (ASES, 2007) to 13–17.5 million GWh/yr
(Chaudhari et al., 2004). Only a fraction of rooftops and other lands can be devel-
oped economically at present for solar-based electricity generation, however; it
is the economics of solar technologies, not the size of the potential resource, that

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significantly limit the ability of solar electricity alone to contribute substantially to
electricity production.
There are two components to the geothermal resource base: hydrothermal
(water heated by Earth) that exists down to a depth of about 3 km, and enhanced
geothermal systems (EGS) associated with low-permeability or low-porosity
heated rocks at depths down to 10 km. There is some potential for expanding
electricity production from the hydrothermal resources and thus affecting regional
electricity generation—for example, a regional study of known hydrothermal
resources in the western states found that 13 GW of electric power capacity
exists in identified resources within this region (WGA, 2006)—but in general, the
resources are too small to have a major overall impact on total electricity genera-
tion in the United States.
It is the heat stored in the low-permeability and/or low-porosity rocks at
great depths that represents the much larger resource base. As noted in a recent
Massachusetts Institute of Technology study, a much larger potential for energy
exists with EGS resources (MIT, 2006). The estimated geothermal resource below
the continental United States, defined as the total amount of heat trapped to
10 km depth, has been estimated to be in excess of 1.3 × 1025 J (MIT, 2006). This
is more than 130,000 times the total 2005 U.S. energy consumption of 1.00 ×
1020 J. However, beyond the total amount of potentially available energy, the rate
of extraction of this energy is especially critical in assessing the actual practical
potential of this energy source. The mean geothermal heat flux at Earth’s surface
is on the order of 50 mW/m2, and in many areas, the geothermal heat flux is sig-
nificantly less than this value. Given that the electrical generation efficiency from
use of this relatively low-temperature heat in a steam turbine is about 15 percent,
the extractable and sustainable electrical power density from the geothermal
resource is on the order of 10 mW/m2. To provide substantial power, heat must
be extracted at rates in excess of the natural geothermal heat flux (heat mining)
in order to usefully tap sufficient geothermal resources. Indeed, the MIT report
(2006) notes that some temperature drawdown should occur if EGS resources
are to be used in their most efficient manner. The substantial technical challenges
associated with tapping this resource are discussed later in this chapter.
Other renewable resources, including conventional hydropower, hydro-
kinetics (wave/tidal/current), and biomass, have significant resource bases, too.
Because the conventional hydroelectric resource is generally accepted to be near
its maximum utilization in the United States, further growth opportunities are
relatively small. Regarding hydrokinetics, one study puts the size of the wave

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energy resource for the East and West Coasts at more than 0.5 million GWh/yr
(EPRI, 2005). This study also estimates the wave energy base in Alaska to be
1.3 million GWh/yr, though it is unclear whether such a resource could be fully
exploited. EPRI (2005) put the capacity of the tidal energy resource at a 152 MW
annual average, which corresponds to an annualized electrical energy production
of 1300 GWh/yr. The biomass resource base is discussed in Chapter 5.
Findings: Resource Base
Solar and wind renewable resources offer significantly larger total energy and
power potential than do other domestic renewable resources. Solar energy is
capable, in principle, of providing many times the total U.S. electricity consump-
tion, even assuming low conversion efficiency. The land-based wind resource also
is capable of making a substantial contribution to meeting current U.S. electricity
demand without stressing the resource base. For these reasons, solar and wind
resources are emphasized, but other non-hydroelectric renewables can make signif-
icant contributions to the electrical energy mix as well, at least in certain regions
of the country. However, renewable resources are not distributed uniformly.
Resources such as solar, wind, geothermal, tidal, wave, and biomass vary widely
in space and time. Thus, the potential to derive a given percentage of electricity
from renewable resources will vary from location to location. Awareness of such
factors is important in developing effective policies at the state and federal level to
promote the use of renewable resources for generation of electricity.
RENEWABLE TECHNOLOGIES
A renewable electricity-generation technology harnesses a naturally existing energy
flux, such as wind, sun, or tides, and converts that flux into electricity. Such tech-
nologies range from well-established wind turbines to pilot-plant hydrokinetic sys-
tems to methods, such as those that exploit salinity and thermal ocean gradients,
that are in the conceptualization or demonstration stages. Some of these tech-
nologies produce power intermittently (technologies that rely on wind and solar
resources), whereas others are capable of producing baseload power (technologies
that rely on hydropower, biomass, and geothermal resources). Though renewable-
electricity technologies show much variability, they do have several shared charac-
teristics: (1) the largest proportions of costs, external energy needs, and material

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inputs occur during the manufacturing and installation stages; (2) there are no
associated fuel costs (except for biomass-fueled electricity generation); (3) oppor-
tunities for achieving economies of scale are greater at the manufacturing stage
than at the generating site—larger-generation units do not necessarily reduce the
average cost of electricity generation as much as they do for coal-fired or nuclear
plants; and (4) renewable electricity technologies can be deployed in smaller incre-
ments and come on line more rapidly.
Technology Descriptions
Wind
Wind power uses a turbine and related components to convert the kinetic energy
of moving air into electricity. A typical wind turbine assembly includes the rotor,
controls, drive train (gearbox, generator, and power converter), other electronics
(wiring, inverters, and controllers), and a tower. Each of these components has
undergone significant development in the last 10 years, and the resulting modi-
fications have been integrated into the latest turbine designs. Critical objectives
for these and future improvements are to make it easier to integrate the wind
power plants into the electrical system and to increase their capacity factors.
Especially important has been the development of electronic controls that allow
modern turbines to remain connected to the electricity grid during voltage dis-
turbances and reduce the draw on the grid’s reactive power resources. Advances
in computerized controls will allow more aspects of the turbine to be monitored,
resulting in more efficient use and the potential to better target and deploy tech-
nical upgrades.
Along with advances in electronics have come improvements in wind turbine
structures, allowing turbine size and generating capacity to grow. Based on the
fact that wind speed increases with height and that energy-capture ability depends
on the turbine’s rotor diameter, the most common turbines at present are three-
bladed rotors with diameters of 70–80 m, mounted atop 60–80 m towers, that
have a capacity of 1.5 MW. The rotor blade has gone through many generations
of designs, using various types of materials and structures, to maximize its aero-
dynamic performance without compromising stability.
Wind power technologies are actively being deployed today, and no major
technological breakthroughs are expected in the near future. However, evolution-
ary modifications in various turbine components are expected to bring 30–40
percent improvement in cost-effectiveness (cost per kilowatt-hour) over the next

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decade (Thresher et al., 2007). And while the turbine tower is not expected to
get much taller, advances will likely occur in installing and maintaining these
machines in difficult-to-reach locations. One possibility, for example, is self-erect-
ing towers. In the future, turbine rotors will be made of advanced materials such
as fiberglass, and they will have improved structural-aerodynamic designs, sophis-
ticated controls, and higher speeds. By reducing the blade-soiling losses (e.g.,
through dust or insect buildup) and installing damage-tolerant sensors and robust
control systems, reductions in energy loss and improvements in turbine availability
can occur. In addition, drive trains will be modified to include fewer gear stages,
medium- and low-speed generators, distributed gearbox topologies, permanent-
magnet generators, and new circuit configurations. As shown in Table 6.1, these
improvements will have significant impacts on annual wind energy production and
capital costs over the next decade. It should be noted that future capital costs also
will be greatly influenced by global supply and demand for wind turbines. Some
of these issues are discussed in the section titled “Deployment Potential” later in
this chapter, as well as in the report by the Panel on Electricity from Renewable
Resources (NAS-NAE-NRC, 2009).
Along with improvements in onshore wind-turbine designs, offshore wind-
turbine technologies will soon be actively enhanced to take advantage of the
abundant U.S. offshore wind-energy resources. The technologies associated with
offshore wind turbines will face fundamentally different challenges, however,
attributable to the difficulties of building and operating turbines in the ocean and
installing and maintaining transmission lines underwater.
Solar Photovoltaic Power
When sunlight strikes the surface of a PV cell, some of the light’s photons are
absorbed. This causes electrons to be released from the cell, which results in a
current flow, namely, electricity. The two main PV technologies entail flat plates,
which consist of crystalline silicon deposited on substrates, and concentrators,
which typically involve lenses or reflectors that, together with tracking systems,
focus the sunlight onto smaller and more efficient cells.
Silicon is used to form semiconductors in PV cells by taking advantage of
the conductivity imparted when impurities (“doping” elements) are introduced.
Because the efficiency of these crystalline PV modules is only 12–18 percent, fur-
ther development is required—not only to increase efficiency but also to lower
production costs (DOE, 2007a).

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pact storage technologies, based on phase-change and thermochemical mecha-
nisms (with higher storage density than water), and materials that replace the
copper and low-iron glass used in today’s collectors. Reductions in the cost of
manufacture, materials, assembly, and shipping weight may be possible through
a shift from metal/glass components to integrated systems, such as those associ-
ated with polymeric materials, that are manufactured using mass-production
techniques.
Geothermal
Geothermal energy can be applied to a variety of end-uses, including agriculture
(mainly greenhouse heating), aquaculture, industrial processes, and space heating
and cooling of buildings.
Direct-use geothermal taps heated groundwater, without a heat pump or
power plant, for the heating of facilities, with the technology generally involv-
ing resource temperatures between 38° and 150°C (Lindal, 1973). Current U.S.
installed capacity of direct-use systems is 620 MWthermal(MWt).13 Municipalities
and smaller communities provide district heating by circulating hot water from
aquifers through a distribution pipeline to the points of use, though this applica-
tion of geothermal energy remains modest, with systems in only seven states.14
The barriers to increased penetration of direct geothermal heating and cooling
systems are the high initial investment costs and the challenges associated with
locating and developing appropriate sites. The resource for direct heating is richest
in the western states.
Geothermal heat pumps have extended the use of geothermal energy into
traditionally nongeothermal areas of the United States, mainly the midwestern and
eastern states. A geothermal heat pump draws heat from the ground, groundwater,
or surface water and discharges heat back to those media instead of into the air.
The available land area and the soil and rock types at the installation site deter-
mine the best solution. Ground-coupled heat pumps are the most common type
used. The efficiency of the heat pump is inversely proportional to the temperature
difference between the conditioned space and the heat source or heat sink. As a
consequence, heating and cooling efficiencies are improved because ground tem-
peratures remain relatively constant throughout the year. The coefficient of perfor-
13Geo-Heat Center, Oregon Institute of Technology; see geoheat.oit.edu.
14See geoheat.oit.edu/directuse/district.htm.

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mance is 3.4–4.4,15 and the annual operating costs are 25–33 percent of compa-
rable fossil-fuel heating costs.
The electric heat pump is standard off-the-shelf equipment, with minor modi-
fications to handle the heat transfer from geothermal fluids or soil. The heat-pump
equipment is located indoors, which reduces maintenance costs, and the more sta-
ble operating temperature and pressure of the compressor give it a longer life than
in air heat pumps. The unique component is the heat-exchange interface with the
soil or with the groundwater. Ground-coupled heat pumps use high-density poly-
ethylene pipe buried either vertically or in horizontal trenches to exchange heat
between a working fluid and the soil. Vertical loops cost more, but they provide
access to more stable deep-soil temperatures and are the only option if land area
is limited. Regulatory requirements for the bore holes vary across the country,
with the primary regulatory issue being the potential for groundwater contamina-
tion. This problem is addressed by grouting the bore hole; the most commonly
used material (bentonite-based grout) reduces heat transfer to the soil, but more
conductive grouts such as cement mixtures and bentonite/sand mixtures provide
superior performance.
Today, the United States has 700,000 installed units—with 8,400 MWt of
capacity delivering about 7,200 GWh/yr16—and there are 1.5 million units world-
wide. The rate of installation is estimated to be 10,000–50,000 units per year. One
barrier to growth is the lack of sufficient infrastructure (i.e., trained designers and
installers) and another is the high initial investment cost compared to conventional
space-conditioning equipment. There are no major technical barriers to greater
deployment.
Biomass
Burning wood to heat U.S. homes currently represents about 1 percent of fuel used
for direct heating of buildings.17 One-half to two-thirds of residential wood com-
bustion in the United States occurs in wood stoves, as opposed to fireplaces (Fine
et al., 2004). Solid fuels include conventional wood logs, which may or may not
15See www.eia.doe.gov/cneaf/solar.renewables/page/heatpumps/heatpumps.html, Tables 3.3
and 3.4. The coefﬁcient of performance is the ratio of heat output per unit of energy input.
16Geo-Heat Center, Oregon Institute of Technology; see geoheat.oit.edu.
17U.S. Department of Energy, Ofﬁce of Energy Efﬁciency and Renewable Energy. 2008 Build-
ings Energy Data Book, downloadable at buildingsdatabook.eere.energy.gov/. This ﬁgure does
not include biomass that is used in electricity generation.

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have been harvested sustainably, and pellets. Advanced biomass-fuel appliances
use pellets, which are produced by compressing woody material that may include
waste wood and sawdust, agricultural wastes, wastepaper, and other organic
materials. Some pellet-fuel appliances can also burn corn kernels, nutshells, and
wood chips. Pellet stoves use electricity to run fans, controls, and pellet feeders.
One of the concerns about solid-fuel combustion for home heating is air pol-
lution. In areas where wood stoves are prevalent, wood smoke is a major source
of fine particulates and gaseous pollutants, including nitrogen oxides, carbon
monoxide, and organics. The mandatory smoke-emission limit set by the U.S.
Environmental Protection Agency (EPA) for wood stoves is 7.5 grams of smoke
per hour for noncatalytic stoves and 4.1 g/h for catalytic stoves.18 Modern noncat-
alytic stoves have improved fireboxes to achieve high combustion efficiency. The
most efficient wood-burning appliances also use catalytic converters to achieve
nearly complete combustion of the feedstock and to reduce harmful emissions.
Stoves are available with EPA-certified emissions as low as 1 g/h. Stoves require
homeowner maintenance and catalyst replacement, however, to retain their high
efficiencies and low emissions.
In summary, modern solid-fuel stoves are efficient and clean compared to the
fireplaces of the past. The economics of using a stove to combust biomass prod-
ucts depends on the fuel being displaced and the distance from home to supplier.
CONCLUSION
A future characterized by a large penetration of renewable electricity represents
a paradigm shift from the current electricity generation, transmission, and dis-
tribution system. There are many reasons why renewable electricity represents
such a shift, including the spatial distribution and intermittency of some renew-
able resources, as well as issues related to greatly increasing the scale of deploy-
ment. Wind and solar—two renewable-energy resources with the potential for
large near-term growth in deployment—are intermittent resources that have some
of their resource bases located far from demand centers. The transformations
required to incorporate a significant penetration of additional renewables include
transformation in ancillary capabilities, especially the expansion of transmission
18See www.epa.gov/woodstoves/basic.html.

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and backup power resources, and deployment of technologies that improve grid
intelligence and provide greater system flexibility. Further, supplying renewable
resources on a scale that would make a major contribution to U.S. electricity
generation would require vast investment in and deployment of manufacturing
and human resources, as well as additional capital costs relative to those associ-
ated with current generating technologies that have no controls on greenhouse gas
emissions. The realization of such a future would require a predictable policy envi-
ronment and sufficient financial resources. Nevertheless, the promise of renewable
resources is that they offer significant potential for low-carbon generation of elec-
tricity from domestic sources of energy that are much less vulnerable to fuel cost
increases than are other electricity sources. Overall success thus depends on having
technology, capital, and policy working together to enable renewable-electricity
technologies to become a major contributor to America’s energy future.
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AWEA. 2009b. AWEA First Quarter 2009 Market Report. Washington, D.C.
Baidya, Roy S., S.W. Pacala, and R.L. Walko. 2004. Can large wind farms affect local
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